Motor current control circuit utilizing real current component

ABSTRACT

A control circuit obtains a signal proportional to current supplied to an induction motor from a variable-frequency inverter. A phase-sensitive detector discriminates between the real and reactive components of the motor current and the real component is fed back to control the current to the motor, preferably in a current limit circuit which limits the torque output of the motor. The foregoing abstract is merely a resume of one general application, is not a complete discussion of all principles of operation or applications, and is not to be construed as a limitation on the scope of the claimed subject matter.

United States Patent Robert G. Schiernnn Cleveland Heights, Ohio 863,922

Oct. 6, 1969 Nov. 9, I971 Reliance Electric Company Inventor Appl. No.Filed Patented Assignee MOTOR CURRENT CONTROL CIRCUIT UTILIZING REALCURRENT COMPONENT Primary Examiner-Gene Z. Rubinson Attorney-Woodling,Krost, Granger and Rust ABSTRACT: A control circuit obtains a signalproportional to current supplied to an induction motor from avariablefrequency inverter. A phase-sensitive detector discriminatesbetween the real and reactive components of the motor current and thereal component is fed back to control the current l5 Chins lo DrawingFigs to the motor, preferably in a current limit circuit which limitsUS. Cl 318/231, the torque output of the motor. The foregoing abstractis 318/227 merely a resume of one general application, is not a completeInt. Cl H02p 5/40 discussion of all principles of operation orapplications, and is Field ol Search 318/227, not to be construed as alimitation on the scope of the claimed 230, 231 subject matter.

:6 3 SP ASE 51, QECT/F/EE ZZ PATENTEUuuv 9 Ian SHEET 2 BF 2 Fig. 4

Y s am Mao aw Fig. 5A ml Fig. 5B

Fig. 50

m T N E V m arm/wens.

MOTOR CURRENT CONTROL CIRCUIT UTILIZING REAL CURRENT COMPONENTBACKGROUND OF THE INVENTION The typical induction motor operation isthought ofas a constant speed operation because of the usual applicationof a constant frequency, e.g., 60 Hz. to the motor resulting in a speedproportional to that frequency and inversely proportional to the numberof the poles in the motor. For variablespeed operation the prior art formany years has used DC motors because of their good variable-speedcharacteristics and good starting torque. However, in many applicationssuch as steel mills, process lines, etc., the atmosphere may be verydusty or corrosive and as a result the DC motor with its commutator andbrushes is not only a maintenance problem but actually hazardous becauseof the arcing at the brushes. The dusty atmosphere causes frequent brushreplacement and even frequent turning down of the commutator. In suchatmosphere and use conditions, the squirrel cage induction motor withits absence of brushes, commutators, and sliprings and its ruggedconstruction is highly desirable. Yet operation from aconstant-frequency source means that the motor has lower startingtorque, high starting currents and essentially a constant-speedoperation.

In recent years operation of the induction motors fromvariable-frequency devices such as cycloconverters and inverters hascome into increasing use in order to obtain a variable speed ofoperation of the induction motor. The typical circle diagrams andequivalent circuit for induction motors found in textbooks and handbooksare approximations at best,and are approximations based on the premiseof operation of the induction motor at a medium frequency for example,50 or 60 Hz. This is because the induction motor has been around fordecades and for all of its early years was considered essentially aconstant-frequency constant-speed device. Now that the motor is beingused in variable-frequency and hence variablespeed applications, it hasbeen found that new problems have arisen in the low end of the speedrange. On inverter drives a speed range of :1 is typical and with pulsewidth modulation techniques the speed range may be 50:1 or even 100:1.This means that a motor with a 1,750 r.p.m. base speed may be operateddown to 175 r.p.m. or even down to as low as 17.5 r.p.m. with PWMtechniques. At this low-frequency low-speed operation, it has been foundthat the motor overheats and can overheat much worse for light loadsthan it does for heavy loads. This seems to be the reverse of what onewould expect, and it has beenfound that this overheating is caused byover excitation. The induction motor typically draws a lagging currentdue to the excitation current required which is a reactive component. Atno load this reactive component is predominant with a real component ofthe current only sufficient to overcome iron and copper losses andwindage and friction. For practical purposes this no load, full-speedoperation of an induction motor is one which draws current almost 90 outof phase with the voltage and in a typical induction motor this totalmotor current value is about 50 percent of nameplate or rated current.In many induction motors the no load current can be 100 percent or evenmore of the rated full load current, all due to the high excitationcurrent required. As the motor is loaded, the real component of thecurrent increases in direct proportion, because the real component ofcurrent is essentially proportional to torque. Accordingly the totalcurrent drawn by the motor lags less and less as the torque loadincreases, and at full load might be in the order of 70 to 85 percentpower factor.

Where the induction motor is being supplied from a variable-frequencydevice such as an inverter, the variable-currentconducting devices suchas thyristors in the inverter, must be protected against overcurrentelse they could burn out. The very fact that the no load current may befrom 50 to 120 percent of nameplate rating, however, makes it apractical impossibility to use the total current signal as a feedback tocontrol or limitthe current in any way.

According, an object of the invention is to control the current in amotor fed from a variable-frequency device.

Another object of the invention is to control the current in aninduction motor supplied by an inverter.

Another object of the invention is to provide a control signal utilizingthe real component of the current in an induction motor as the controlfor the current supplied by an inverter to such motor.

Another object of the invention is to provide a current limit signalwhich controls both the voltage and frequency of an inverter supplyingcurrent to a motor.

Another object of the invention is to provide a current limit circuitwhich limits the current on both motoring and regenerating actions ofthe motor.

Another object of the invention is the provision of a currentcontrolling signal utilizing the real component of the current to lowerthe output voltage of the inverter supplying current-to the motor duringmotoring action and to raise the output voltage of the inverter duringregenerative action of the motor.

Another object of the invention is to provide a control circuit whichcontrols and limits the torque output of a motor supplied by avariable-frequency device.

Another object of the invention is to provide a phasesensitive circuitfor a motor which is responsive to only the real component; that is,in-phase or 180 out-of-phase (regenerative) component of the motor loadcurrent.

Another object of the invention is to provide a phase-sensitive detectorto detect between the in-phase component and the reactive component ofmotor load current.

SUMMARY OF THE INVENTION The invention may be incorporated in a controlcircuit comprising, in combination, a motor, a variable-frequency deviceconnected between voltage source terminals and said motor to supplyenergy of a variable current to said motor, regulator means connected toregulate the output of said variablefrequency device, means connected tosense the current in said motor, phase-sensitive detector meansconnected to said sensing means to discriminate between the real andreactive components of the current, and control means connected to beresponsive to the real component of current output of said detectormeans and connected to said regulator means to control the currentoutput of said variable-frequency device.

Other objects and a fuller understanding of the invention may be had byreferring to the following description and claims, taken in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of amotor circuit utilizing the invention;

FIGS. 2 and 3 are graphs of operating characteristics of the circuit;

FIG. 4 is a schematic diagram of a preferred embodiment ofphase-sensitive detector means; and

FIGS. SA-SF are graphs of currentsexplaining operation of the circuit ofFIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 shows a schematic diagramof a motor circuit 11 incorporating a preferred embodiment of theinvention, however, it will be understood from the entire disclosureincluding the claims that the invention is not limited to thisparticular form shown in FIG. I. This motor circuit 11 includesgenerally a motor 12 shown as an induction motor and this may be asquirrel cage motor for ruggedness. Also included'is avariable-frequency device 13 which may be a cycloconve'rter, forexample, but=preferablyis an inverter. The inverter 13 is sup- .pliedwith energy from a voltagesource 14 shown as athreephase sourcesupplying energy via a rectifier 15 which in turn supplies directcurrent energy on conductors 16 to the inble conducting devices such astriacs or thyristors 18 to selectively control the frequency and thevoltage of the energy supplied on conductors 19 to the motor 12. In thisembodiment these conductors 19 are shown as supplying three-phase energyto the motor 12. The inverter 13 has a frequency control terminal 20 anda voltage control terminal 21 supplied with appropriate signals from aregulator 23. The inverter 13 may be any one of several differentconventional types for example, it may be that shown in the MokrytzkiU.S. Pat. No. 3,391,328 issued July 2, 1968; that in the Mokrytzkiapplication Ser. No. 624,539 filed Mar. 20, 1967 for Pulse WidthModulated inverter; or that in the Hammond application Ser. No. 755,927filed Aug. 28, 1968, entitled Synchronizing Cir cuit. In general theinverter 13 has a frequency control signal applied to the controlterminal 20 in order to control the frequency of the output of theinverter on the conductors 19 and hence control the speed of operationof the motor 12. Also a voltage control signal applied to the controlterminal 21 controls the magnitude of the output voltage and hencecontrols the torque or load-carrying capabilities of the motor 12.

The motor circuit 11 also includes a phase-sensitive detector means 25having a feedback 26 from the conductors 19 or the motor 12 sensing theamount of motor current. The phasesensitive detector means 25 detectsthe real component of the motor current, that is, either the in-phase ofthe 180 out-ofphase component of the motor load current anddistinguishes this from the reactive component of the load current. Thedetector means 25 has an output on conductor 27 to control means 28.This control means 28 includes an 1R drop compensation signal suppliedon conductor to the regulator 23. The real component of the current issupplied on conductor 27 to the control means 26 wherein it passesthrough an invertcr 29 to a summer 31). After the summer it is fed to acurrent limit circuit 31. The current limit circuit is fed by a constantcurrent source 32. The output of the current limit circuit appears onconductor 33 and is fed to an output amplifier 34. A reference source 38is also supplied to this output amplifier 34 and this may be a volts percycle reference source to keep substantially constant voltage per cycleof the inverter 13. The output of the output amplifier 34 appears on aconductor 39 which is fed to the regulator 23. The regulator controlsthe frequency of the inverter 13 at control terminal 20 and controls thevoltage output of the inverter at control terminal 21.

The current limit circuit 31 is a means to control the current in themotor 12 in accordance with the real component of such current. In thispreferred embodiment the control of the current is a limiting of themaximum value of such current and accordingly a limiting of the torqueoutput of the motor. This protects the motor and the load which itdrives and also protects the thyristors 18 within the inverter 13 fromoverload currents.

The signal coming from the phase-sensitive detector line 27 is a DCsignal which is increasing negative for increasing currents to the motor12, as will appear below. Accordingly, the inverter 29 merely changesthe sign of the signal to have a positive DC signal fed to the summer30. An antihunt circuit 55 also connected to be summed at summer 30.This signal from the summer 36 passes through an input resistor 41 or aresistor 42 depending upon whether the motor 12 is motoring orregenerating at the time. For motoring action the current is negativeand is passing through resistor 42. The constant current source 32supplies a constant positive current on conductor 44 and a constantnegative current on conductor 43, all as set by rheostat 45 whichdetermines the point at which the motor current is limited. Thisconstant current from the source 32 passes through diodes 48 and 47 inseries which normally bias off the transistors and 49. For example, ifresistor 42 has a value of 1,200 ohms and rheostat 45 has been set sothat there is l milliampere flowing through the diodes 48 and 47, when lmilliampere flows through resistor 42, this will turn on the transistor50 to cause a voltage drop across an output resistor 52. This willsupply a negative voltage output on the conductor 33 which will decreasethe positive voltage from the maximum speed potentiometer 56. Thedecreasing negative voltage is inverted and passed by amplifier 34 toregulator 23. The regulator is such that for this decreasing negativesignal input, the inverter 13 has a decreasing voltage output to thuslimit the current supplied to the motor 12.

For regenerating action the signal on conductor 27 will be positive andbe inverted by amplifier 29 to be negative as supplied to amplifier 30.The positive signal out of amplifier 30 would now pass through resistor41 when it exceeded 1 milliampere, for example, this would turn ontransistor 41 and a voltage signal would appear across a resistor 51. Inthis case it is a positive signal which increases the positive signalfrom potentiometer 56 which is inverted by amplifier 34 to be anincreasing negative signal on conductor 39 to the regulator 23. Thisincreasing negative signal on the regulator increases the output voltageon the inverter so that the current fed back from the motor 12 duringits regenerative action is opposed by the higher voltage of the inverterand therefore the regenerative current is limited at the preset value.

A switch 59 optionally may be closed to bypass current through aresistor 60 around the current limit circuit 3]. This providescontinuous current regulation, which is used on torque sharing of manymotors driving a common load.

The constant current source 32 is a reference source and limit rheostat45 sets the adjustable value of current through diode 47 to the base orinput of amplifier or transistor 49. This reference value is comparedwith the input to the transistor 49 from the real component of the motorcurrent through input resistor 41. When the real component is in excessof the reference value, then an error signal is developed, transistor 49is no longer biased off, and its conduction through output resistor 51is the spillover or output of the control means 28 to the regulator 23.

The phase-sensitive detector means 25 is sensitive to whether the motor12 is operating as in induction motor or whether there is regenerativecurrent flowing and this motor is acting as an induction generator.Accordingly, the phase-sensitive detector means 25 senses the netin-phase current or the net l out-of-phase current, that is, it sensesthe real component of the current as distinguished from the reactiveportion of this current.

Frequently in a variable-frequency variable-voltage AC drive system suchas the motor circuit 11, the applied voltage on conductors 19 isadjusted to provide a constant volts per cycle ratio, which in an idealmotor produces constant flux in the air gap.

it has been found that the real component or the in-phase component ofthe motor current is directly proportional to the torque of the motorand accordingly this real component of the motor current is used as acontrol signal to control or limit current or torque. Thisphase-sensitive detector means 25 monitors the motor current andgenerates a signal proportional to the real component of the loadcurrent.

The phase-sensitive detector means 25 may take a number of formsincluding single and polyphase versions with more or less precision,depending upon the degree of speed or accuracy required. One version isthe preferred embodiment shown in FIG. 4. This detector means circuit 25shows in FIG. 4 includes phase detector circuits 61, 62 and 63 for eachof the three phases of the polyphase source, shown as three-phase. Thephase detector circuits 61 and 62 may be identical and only circuit 61will be described in detail. Terminals 64 and 65 are phase inputterminals carrying current proportional to and directly in phase withcurrent on two of the conductors 19 to the motor 12. This may beobtained in a number of ways, for example, by use of a small currenttransformer on each of the conductors 19, or by Hall effect transducers.Each phase detector circuit 61 and 62 has a first path 67 and a secondpath 68 leading to a common terminal 69. The first path 67 includes aresistor 71 and the second path 68 includes a second resistor 72 ofone-half the ohmic value of resistor 71. Preceding this resistor 72there is a unity gain inverting operational amplifier 73 which isinverting because of the input to the negative terminal and is unitygain because an input resistor 74 has the same resistance value as afeedback resistor 75. Parallel and oppositely connected diodes 77protect an FET switch 78 connected in series in the path 68. The switch78 has a gate 79 connected to be triggered into conduction by a ringcounter 80 in synchronism with the phase voltage for that particularphase.

The phase-sensitive detector means 25 also includes an invertingamplifier 81 which amplifies the DC component of the current appearingon terminal 69. A filter capacitor 82 smooths the output voltage of theamplifier 81 appearing on the compensation signal output terminal 27 andfeedback resistor 83 sets the gain of the amplifier 81 which may be alow gain, for example, unity gain.

The phase detector circuit 62 may be identical to the phase detectorcircuit 61. The phase detector circuit 63 is difierent from circuits 61and 62 because in this preferred embodiment only two current-sensingdevices are used on phases A and B of the three conductors leading toinduction motor 12. The signal proportional to the current in the thirdphase is artificially created in the phase detector circuit 63 in orderto save the cost of another current transducer at the motor inputconductor 19. This artificial creation of the current signal is basedupon the fact that the algebraic sum of all currents in the threeconductors 19 must be zero at any given instant. Accordingly, thealgebraic sum of the currents in phases A and B must always equal theinverse of the current in phase C. Accordingly the signals from thecurrent phase terminals A and B are passed through input resistors 86 toa unity gain amplifier 87 whereat the two signals are summed andinverted and the phase signal I accordingly appears at output terminal88. This signal then passes through resistor 71 to the common terminal69. At the same time the phase current signals A and B are passedthrough input resistors 89 each having a value of R/2 and are passed tothe FET switch 78. Accordingly this phase detector circuit 63 operatesin the same way as detector circuits 61 and 62 as shown below.

The operation of the phase-sensitive detector means 25 of FIG. 4 may beexplained by use of the current diagrams of FIG. 5. Considering just asingle phase detector circuit 61 and assuming for the moment that adirect current is flowing through the two paths 67 and 68, one willobserve that in the first path 67 a current will flow equal to Elk.Because resistor 72 has only one-half the resistance value of resistor71, then in this second path 68 a current will flow equal to -2E/R.Accordingly FIG. 5A shows a steady DC current 86 equal to E/R will flowin path 67 under this hypothetical situation of a direct current flow.In the second path 68 a current I= -2E/R as shown by curve 87 will Howand this is negative because of the inverting amplifier 73. It isassumed that the current flows only half the time; that is, the switch78 is open half the time and closed half the time. FIG. 5C shows aresulting curve 88 of a combination of curves 86 and 87 occurring at thecommon terminal 69 which results from a summation of the currentsthrough the two paths 67 and 68. This is a straight a algebraicsummation and it shows that the current alternates from a minus to aplus one unit value with the intervals of negative and positive beingequal.

With this simplified explanation, next consider FIGS. SD, SE and SFwhich show a sinusoidal current flow. FIG. 5D shows a curve 90A ofcurrent flow through the first path 67, assuming a zero phase anglebetween the phase current and the phase voltage such as E,,, whichtriggers the gate 79 of the FET switch 78. FIG. 55 shows a curve 90B ofthe inverted and doubled current in the second path 68 for the same zerophase angle. FIG. 5F shows a composite curve 90C which is a summation ofthe two currents at the terminal 69 of the two curves 90A and 908. Itwill be noted that in each half cycle the current curve 90C is a maximumnegative and accordingly when inverted by the inverting amplifier 81will appear as a maximum positive compensation signal at terminal 27.

Next consider a current curve 91A in path 67 which is a 45 laggingcurrent such as is commonly incurred in induction motor operation as aninduction motor. Curve 915 in FIG. 5B

shows the current through path 68 and curve 91C in FIG. 5F shows thecomposite current 91C of the current at terminal 69. It will be notedthat the average negative current is less than that of the negativecurrent for curve C. Next for a 90 lagging current such as occurs duringidling of an induction motor with just the magnetization current and nowindage and friction losses, then a curve 92A in FIG. 5D shows thecurrent through path 67. FIG. 5E shows a curve 92B of current throughpath 68 and FIG. 5F shows a curve 92C of the composite current atterminal 69. This is a curve which is equal on both the positive andnegative sides of the zero axis and hence when filtered by the capacitor82, there will be a zero voltage appearing as a compensation signal atthe terminal 27. Next consider when the induction motor is regeneratingand acting as an induction generator then the current will lag 135, forexample, and a curve 93A will be typical for the current through path67. Current curve 933 will show the current flow through path 68 andcurrent curve 93C will show the composite current at terminal 69. Thisis a positive average voltage and when inverted by inverting amplifier81 it will appear as a definite negative compensation signal at theterminal 27. This shows that the control signal changes sign at theproper time; namely, the changeover from motoring action to regenerativeaction. Accordingly the control is positive during regeneration and isnegative during motoring.

The motor circuit 11 has some means to sense the current in the motor 12and this is shown as current transducers 93 and 94 on two of theconductors 19. These may be current transformers, for example, but inview of the changing frequency on these conductors which may go to verylow values, for example, l or 2 Hz., it is preferred to use Hall effecttransducers. Such devices have a voltage input at 95 which, for example,might be 60 bolts at600 Hz., and this combined with the field in thecore which is proportional to the current flow in conductor 19, willestablish an output voltage on the conductor 26 proportional to suchmotor current. Such Hall effect current transductors are commerciallyavailable and one such suitable unit is Reliance Part No. B/n 05 1 368.

FIG. 3 shows a curve 101 which is a plot of volts versus the realcomponent of the current. This is the output voltage of the inverter 13and in this preferred embodiment the frequency is also proportional tothe voltage in order to maintain a constant volts per cycle ratio. Theoutput voltage remains constant until a limit point 102 is reachedwhereat the current limit circuit 31 is brought into play. The currentis accordingly limited and drops down toward zero because the outputvoltage of the inverter 13 decreases along a curve 103. This is not avertical curve in order to maintain stability by means of the antihuntcircuit 55. The steepness of such curve 103 may be changed by theposition of the rheostat 54 in the antihunt circuit 55 which feedsenergy back from the amplifier 34 to the input of summer amplifier 30.

One use of this motor circuit 11 might be in a steel mill, for example,wherein a steel billet is to be accelerated as rapidly as possible on arunout table in a blooming mill. The steel billet or bloom is rapidlyaccelerated, passed through the blooming mill and then decelerated,reversed in direction and again passed through the blooming mill. Formaximum production of the steel mill, the steel billet has to beaccelerated and decelerated as rapidly as possible yet for economy themotor cannot be oversized nor can the inverter components be oversized.The present invention permits controlled acceleration and decelerationat the maximum rate consistent with the torque capabilities of the motorand the current-carrying capabilities of the thyristors in the inverter13. FIG. 2 shows the typical speedtorque curve 106 for an inductionmotor with a current limit established by a vertical line 107. Thismight be percent of rated current and torque or might be percent on amotor to drive a web of paper in a process line which could easily tear,or might be percent on a blooming mill drive where the duty cycle wassufficiently long to permit short periods of current greater than rated.The FIG. 2 also shows an entire family of speed-torque curves, and

starting from rest if full voltage were somehow supplied to the invertercontrol terminal 21, the current would be limited to a point 108 on thecurrent limit line 107 with the voltage and the frequency of the outputof the inverter 13 controlled at a low value. This might be a frequencyof 2 Hz. for low slip-controlled acceleration of the motor. As the motoraccelerated under full current and torque conditions, the frequency andvoltage would both be increased to perhaps Hz. at a point 109. Thefrequency would continue to increase through the family of speed-torquecurves all the way to a maximum for example, 60 Hz. or to whatevermaximum frequency was desired as set by a maximum frequency and maximumspeed potentiometer 56. The speed-controlling potentiometer 57 maysupply a signal through a linear voltage with time unit 58 whichprovides an acceleration control signal to the amplifier 34.

During controlled rapid deceleration of the motor 12, for example, wherea steel billet on a runout table is attempted to be rapidly decelerated,the motor can regenerate and pump power back to the inverter 13. In suchcase the current can attempt to exceed the current or torque set by thelimit potentiometer 455. In such case a positive DC signal is passed bythe phase-sensitive detector 25 on conductor 27. This becomes negativeat the output of amplifier 29 and positive at the output of amplifier30. This positive signal is passed through resistor 41 to causetransistor 47 to conduct and accordingly an increasing positive signalis applied on conductor 33 to amplifier M. An increasing negative signalis then passed to the regulator 23 and this negative signal is passed tothe voltage control terminal 21 on the inverter 13 to increase thevoltage output thereof. The current being pumped back from motor 12 intothe inverter 13 thus encounters this higher voltage to limitregenerative current supplied by the motor 12.

The present control circuit utilizing only the real component of themotor current rather than the total current makes practical a torquelimit circuit for the motor 12 operating from a variable-frequencydevice 13. As stated above, a typical induction motor may have a totalmotor current of about 50 percent of rated current when operating at noload. Some induction motors have 100 percent, or more of rated currentwhen operated at no load. Under such conditions, it would be impracticalto attempt to limit the current to a motor by using the total motorcurrent as a signal. By using the real component of the current, eitherdirectly in-phase or 1 80 outof-phase relative to the applied voltage,this practical difficulty is obviated.

The present disclosure includes that contained in the appended claims,as well as that of the foregoing description. Although this inventionhas been described in its preferred form with a certain degree ofparticularity, it is understood that the present disclosure of thepreferred form has been made only by way of example and that numerouschanges in the details of the circuit and the combination andarrangement of circuit elements may be restored to without departingfrom the spirit and scope of the invention as hereinafter claimed.

What is claimed is:

l. A control circuit comprising in combination,

a motor,

a variable-frequency device connected between voltage source terminalsand said motor to supply energy of a variable current to said motor,

regulator means connected to regulate the output of saidvariable-frequency device,

means connected to sense the current in said motor,

phase-sensitive detector means connected to said sensing means todiscriminate between the real and reactive components of the current,

and control means connected to be responsive to the real component ofcurrent output of said detector means and connected to said regulatormeans to limit the current output of said variable-frequency device.

2. A circuit as set forth in claim 1, wherein the limited current outputlimits the torque applied by the motor.

3. A circuit as set forth in claim 1, wherein the real component of themotor current is limited.

4. A circuit as set forth in claim 1, including means to vary thevoltage output of said variable-frequency device, and said control meanscontrols the voltage output of said variablefrequency device.

5. A circuit as set forth in claim 4, wherein said control meanscontrols the voltage and frequency output of said variable-frequencydevice.

6. A circuit as set forth in claim 1, wherein said variablefrequencydevice is an inverter.

7. A circuit as set forth in claim 1, including a reference source, andmeans comparing said real current component with said reference sourceto obtain an error signal,

said error signal controlling said regulator means.

8. A circuit as set forth in claim 1, said control means controls thefrequency of said variable-frequency device.

9. A circuit as set forth in claim 1, wherein said current is controlledduring both motoring and regenerative action of said motor.

10. A circuit as set forth in claim 1, wherein said motor is aninduction motor.

11. A circuit as set forth in claim 1, including a current referencesource,

means comparing the real component of the motor current with saidreference source,

said control means being responsive to the excess of said real componentover said reference source.

12. A circuit as set forth in claim 1, wherein said control meansincludes a reference source,

an amplifier having an input and an output,

an output resistor connected to the output of said amplifier,

means connecting said reference source to said amplifier,

and means connecting said real component of the motor current to saidamplifier input,

whereby when the input current to said amplifier exceeds a value presetby said reference source the amplifier is biased into conduction todevelop a voltage across said output resistor for an output from saidcontrol means.

13. A circuit as set forth in claim 1, wherein said control meansincludes a current source,

a transistor having an input and an output,

an output resistor connected to the output of said transistor,

a diode connected between said current source and said transistor input,

an input resistor connected to said transistor input,

and means connecting an input from said real component of the motorcurrent to said input resistor,

whereby when the input current through said input resistor exceeds avalue preset by said current source the transistor is biased intoconduction to develop a voltage across said output resistor for anoutput from said control means to said regulator means.

14. A circuit as set forth in claim 1, wherein said control meansincludes a current limit having a constant current source,

a diode connected across said constant source output,

a transistor having an input and an output,

an output resistor connected to the output of said transistor,

means connecting said input of said transistor to said diode to benormally biased thereby into a nonconducting state, an input resistorconnected to said transistor input,

and means connecting an input from said real component of the motorcurrent to said input resistor,

whereby when the input current through said input resistor exceeds avalue preset by said constant current source the transistor is biasedinto conduction to develop a voltage across said output resistor for anoutput from said current limit circuit.

15. A circuit as set forth in claim 1, wherein said control meansincludes a current limit circuit having a constant source havingnegative and positive current outputs,

first and second diodes connected in series across said positive andnegative constant current source outputs,

the junction of said input resistors for current flow through said firstor second input resistors dependent upon whether the real component ofthe current is positive or negative,

whereby when the input current through one of said input resistorsexceeds a value preset by the constant current source the respectivetransistor is biased into conduction to develop a voltage across therespective output resistor for an output from said current limitcircuit.

I t I F It

1. A control circuit comprising in combination, a motor, avariable-frequency device connected between voltage source terminals andsaid motor to supply energy of a variable current to said motor,regulator means connected to regulate the output of saidvariable-frequency device, means connected to sense the current in saidmotor, phase-sensitive detector means connected to said sensing means todiscriminate between the real and reactive components of the current,and control means connected to be responsive to the real component ofcurrent output of said detector means and connected to said regulatormeans to limit the current output of said variable-frequency device. 2.A circuit as set forth in claim 1, wherein the limited current outputlimits the torque applied by the motor.
 3. A circuit as set forth inclaim 1, wherein the real component of the motor current is limited. 4.A circuit as set forth in claim 1, including means to vary the voltageoutput of said variable-frequency device, and said control meanscontrols the voltage output of said variable-frequency device.
 5. Acircuit as set forth in claim 4, wherein said control means controls thevoltage and frequency output of said variable-frequency device.
 6. Acircuit as set forth in claim 1, wherein said variable-frequency deviceis an inverter.
 7. A circuit as set forth in claim 1, including areference source, and means comparing said real current component withsaid reference source to obtain an error signal, said error signalcontrolling said regulator means.
 8. A circuit as set forth in claim 1,said control means controls the frequency of said variable-frequencydevice.
 9. A circuit as set forth in claIm 1, wherein said current iscontrolled during both motoring and regenerative action of said motor.10. A circuit as set forth in claim 1, wherein said motor is aninduction motor.
 11. A circuit as set forth in claim 1, including acurrent reference source, means comparing the real component of themotor current with said reference source, said control means beingresponsive to the excess of said real component over said referencesource.
 12. A circuit as set forth in claim 1, wherein said controlmeans includes a reference source, an amplifier having an input and anoutput, an output resistor connected to the output of said amplifier,means connecting said reference source to said amplifier, and meansconnecting said real component of the motor current to said amplifierinput, whereby when the input current to said amplifier exceeds a valuepreset by said reference source the amplifier is biased into conductionto develop a voltage across said output resistor for an output from saidcontrol means.
 13. A circuit as set forth in claim 1, wherein saidcontrol means includes a current source, a transistor having an inputand an output, an output resistor connected to the output of saidtransistor, a diode connected between said current source and saidtransistor input, an input resistor connected to said transistor input,and means connecting an input from said real component of the motorcurrent to said input resistor, whereby when the input current throughsaid input resistor exceeds a value preset by said current source thetransistor is biased into conduction to develop a voltage across saidoutput resistor for an output from said control means to said regulatormeans.
 14. A circuit as set forth in claim 1, wherein said control meansincludes a current limit circuit having a constant current source, adiode connected across said constant current source output, a transistorhaving an input and an output, an output resistor connected to theoutput of said transistor, means connecting said input of saidtransistor to said diode to be normally biased thereby into anonconducting state, an input resistor connected to said transistorinput, and means connecting an input from said real component of themotor current to said input resistor, whereby when the input currentthrough said input resistor exceeds a value preset by said constantcurrent source the transistor is biased into conduction to develop avoltage across said output resistor for an output from said currentlimit circuit.
 15. A circuit as set forth in claim 1, wherein saidcontrol means includes a current limit circuit having a constant currentsource having negative and positive current outputs, first and seconddiodes connected in series across said positive and negative constantcurrent source outputs, the junction between said diodes being grounded,first and second transistors, first and second output resistorsconnected in series between said transistors, the junction between saidoutput resistors being grounded, each of said transistors having aninput connected across said series-connected diodes to be normallybiased into a nonconducting state, first and second input resistorsconnected in series across said first and second diodes, an input fromsaid real component of the motor current to the junction of said inputresistors for current flow through said first or second input resistorsdependent upon whether the real component of the current is positive ornegative, whereby when the input current through one of said inputresistors exceeds a value preset by said constant current source therespective transistor is biased into conduction to develop a voltageacross the respective output resistor for an output from said currentlimit circuit.